1. Field of the Invention
The field of the invention is marine thrusters. More specifically, the present invention is directed to a marine thruster that is cavitation resistant and does not require a duct through the hull of the marine vessel to which it is attached.
2. Description of the Prior Art
The prior art includes my U.S. Pat. No. 4,055,947, Hydraulic Thruster, granted Nov. 1, 1977 and my U.S. Pat. Nos. 4,137,709 and 4,213,736, both titled Turbomachinery and Method of Operation, granted Feb. 6, 1979 and July 22, 1980, respectively, which are incorporated herein by reference. The basic design relationship for turbo-machinery is defined by the Euler turbine equation, a form of Newton's laws of motion applied to fluid traversing a rotor, see generally, Shepard, Principles of Turbomachinery, Energy Transfer Between a Fluid and a Rotor (Macmillian Co. 1965).
Most of the marine thrusters presently used on ships and barges require internal ducting through the bow of the ship. These hull ducts are expensive, inconvenient and inefficient. One of the drawbacks with these ducts is that large scooped ports must be made on the side of the ship for the thruster to operate properly. These large ports create tremendous drag as the ship travels through the water. The extra drag has currently become a concern among shipbuilders and users as a result of the high cost of fuels.
The propellers currently used in the ducted thrusters are generally birotational and are prone to cavitation when driven at high thrust levels. The cavitation, besides creating a noise nuisance, is damaging to parts and limits the maximum thrust level, resulting in inefficient operation. Further, as these thrusters are bidirectional, screening is desirable on the intake/outflow ports. The fixed screens on the ports further reduce the maximum thrust.
As the use of the ducted thrusters require a duct from one side of the hull to the other, certain vessels are unable to employ these thrusters. On some vessels it is either too expensive to install the lengthy duct necessary or the length of the duct will require too large a thruster to overcome frictional losses and still achieve adequate thrust. Further, on barges and cargo carriers the duct takes up precious space that would otherwise be used for cargo.
As such, although thrusters are presently available they are extremely inefficient as a result of the drag from the ports, the cavitation of the impellers, losses due to screens and losses due to the length of the duct.